Known in the present state of the art is a monolithic filter on a crystalline plate from La--Ga silicate (LGS) made on a straight XY-cut. Relative dimensions of the exciting electrodes are in this case as follows: ##EQU1## where Lx is the electrode dimension along electric axis X, m;
Lz is the electrode dimension along electric axis Z, m; PA1 Lz is the thickness of the piezoelectric-crystal plate, m. PA1 (cf. the textbook "Monolithic filters and resonators on a new Ga--La silicate piezoelectric". In: Electronic engineering. Series "Radioelectronic parts and components", Issue 2 (63), 1986, p.83 (in Russian). PA1 Lz is the width of exciting electrodes along Z axis, m; PA1 H is the thickness of the piezoelectric-crystal plate, m; PA1 (cf. SU, A, #1,780,147, IPC H03H 9/56, 1990). PA1 H is the thickness of the piezoelectric-crystal plate, m; PA1 S is the area of the exciting electrodes, m.sup.2.
A disadvantage inherent in the known filter resides in a relatively low value of a medium-passband frequency of the filter passband which is not in excess of 20 MHz, this being due to unoptimally selected dimensions of the electrodes with respect x to thickness of the piezoelectric-crystal plate.
One more prior-art monolithic crystal filter is known to comprise at least two acoustically coupled resonators, either of which is established by two overlap exciting electrodes situated on the major faces of a piezoelectric La--Ga silicate wafer of the YXl/.+-..beta..degree. cut, the axis of acoustical coupling therebetween is arranged along the length of the piezoelectric-crystal plate. An angle between the normal to the major wafer face and its mechanical axis Y is selected to be 1.degree. 50'.+-.1.degree., and the length and width of rectangular-shaped exciting electrodes are selected, respectively, from the following relations: ##EQU2## where Lx is the length of exciting electrodes along X axis, m;
A disadvantage of the known filter is a relatively low value of a medium-passband frequency of the filter passband which is not in excess of 20 MHz, this being due to the fact that preselected ratio between the length and width of exciting electrodes, and the thickness of the piezoelectric-crystal plate makes it impossible to realize a technologically small interelectrode gap which is liable to decrease significantly with a frequency raise and with the aforementioned dimensional relations of electrodes; In addition, with a frequency raise discrimination of the filter decreases due to an increase in the level and number of side passbands. Moreover, the aforesaid angle between the normal to the face of the major piezoelectric-crystal plate and its mechanical axis Y equal to 1.degree. 50'.+-.1.degree., fails to provide high temperature stability of the filter medium passband frequency in a wide range of above-zero temperatures.